Laser beam emitting devices, for use as laser beam sources in laser beam printers or the like, produce a laser beam to form desired images on a charged photoconductive drum. The drum then rotates; picking up toner to develop a toner image. The toner image is then transferred to a recording sheet or medium, such as paper through a heating process. The image is then fixed on the recording medium. The diameter of the dots that form the image on the recording sheet corresponds to the diameter of the laser beam. If a constant output power level laser beam is employed to generate dots on a sheet, then since the dots that are printed on a sheet are of the same diameter, they may overlap each other resulting in the printed characters or images not being sharply defined; this problem is particularly acute for very small or complex character or image patterns that are printed on the sheet.
One recent laser beam emitting device is capable of emitting different laser beam output power levels and is capable of selecting one of the output power levels depending on the size or complexity of a character or image pattern which is to be printed. If tile character or image pattern to be printed on a sheet is smaller or more complex, a smaller laser beam output power is selected to produce smaller dots on a sheet.
There has been proposed a control device for use with a driver for energizing a laser diode used as a laser beam source in a laser beam emitting device. The control device outputs a control signal which controls the driver to select one of a plurality of different currents to be supplied to the laser diode so as to produce properly sized dots for the character or image pattern to be printed.
However, the proposed control device has the disadvantage that a certain time delay is introduced when the driver selects one of the different currents to be supplied to the laser diode, and the laser beam output power level produced by the laser diode tends to become unstable during the time period immediately after the driver has selected one of the different currents. During such a time delay or period of laser beam output power level instability, the laser beam emitting device is incapable of emitting a laser beam at the desired output power level. Therefore, when the laser diode is frequently switched to different currents during high-speed printing, for example, the laser beam emission from the laser diode cannot respond quickly to the control signal from the control device. Therefore the speed at which printing may be done must be reduced. As a consequence, the laser beam emitting device is unable to quickly produce, the desired dot diameters, and thus high-quality printed copies cannot be produced, especially during high-speed printing.